Currently, optical fibers used for optical fiber communication systems which are generally spreading adopt a structure in which an outer periphery of one core is surrounded by a clad, and information is transmitted when an optical signal propagates in this core. Further, as the optical fiber communication systems spread, the amount of information to be transmitted is dramatically increasing in recent years. Following an increase in the amount of information to be transmitted, the optical fiber communication systems use several tens or several hundreds of multiple optical fibers to perform long-distance optical communication of a large volume.
It is known that, to reduce the number of optical fibers in such an optical fiber communication system, a plurality of signals is transmitted by means of light propagating in respective cores using a multi-core fiber in which outer peripheries of a plurality of cores is surrounded by one clad.
In the optical fiber communication systems, when long-distance optical communication is performed, a plurality of optical fibers may be connected. Further, when the multi-core fiber is used, a plurality of multi-core fibers may be connected. When the plurality of multi-core fibers is connected, it is preferable for specific cores of the multi-core fiber are connected to each other.
However, in the multi-core fiber, by the demand for homogeneity of an optical property of the cores, the cores are arranged so as to be symmetric with respect to a central axis of the fiber in some cases. However, when the cores are arranged in symmetric with respect to the central axis of the fiber, if the multi-core fibers are connected to each other, it is difficult to identify specific cores and thus it is difficult to connect specific cores.
In Patent Document 1, it is disclosed that in order to easily connect multi-core fibers, the multi-core fiber has a marker which is formed of a quartz glass having a high refractive index or a quartz glass having a low refractive index in a clad to be parallel to the core.
[Patent Document 1] WO 2010/073821 A